This invention relates to a method for producing power semiconductor devices, and more particularly, to such a power MOSFET fabrication process and structure which substantially eliminates the incidence of fatal defects in a power device as a consequence of defects in, and/or misalignment between, the layers used in the production of such devices.
In the prior art fabrication of transistor devices on silicon wafers, such as power-MOS field-effect transistors, there have been significant problems in (1) obtaining an acceptably high yield of relatively large-current-capability transistors without (2) driving the cost of production to extremely high and unacceptable levels. A major contributor to this problem has been that the best known prior art fabrication techniques for making power MOSFET devices typically employ five or more independent masking, diffusion and metallization steps, each offering a significant opportunity for the creation of a fatal error in a device. Generally speaking, the more steps, the greater the likelihood of fatal defects; or conversely, the lower the yield of devices that operate within specifications. In high-current/high voltage power devices, it is especially important to avoid designs and defects that can lead to current leakage, shorting, high on-resistance or a wide variation in performance characteristics among nominally similar devices.
One cause of these defects is misalignment occurring during successive masking steps. Defects can also occur in situations where one or more of the masks or layers may, individually, have localized defects. Also, fatal defects can occur if airborne contaminants collect on a mask or a wafer. This possibility is also aggravated by the plurality of masking steps now required.
Gate and source contacts have been conventionally separated using masking or multiple layer techniques. These steps require critical alignment and/or an intermetallic dielectric such as oxide, PSG, BPSG, BSG, or other material such as polyimide. This approach, though effective and traditional, is complex and lends itself to excessive yield loss and cumbersome processing techniques. Also, the device structures that result from these techniques have a tendency to increase certain undesirable parasitic effects.
These problems make it difficult economically, with any expectation of achieving an acceptably high yield, to manufacture relatively large, high-current-capability devices. Put another way, the larger the design of the device, the greater is the likelihood that it will contain a fatal defect. To date, an economically practical size limit has been about 0.25 inches on each side of a device. Accordingly, the tendency in the past has been to reduce the size of individual devices to increase the chances of a larger number of smaller devices surviving defects. However, these smaller devices, while emerging with an acceptable yield percentage, are capable only of handling relatively low-level currents, and thus low-power applications. Accordingly, they must be linked electrically in collections in some fashion in order to be able to handle relatively high-power applications.
Past efforts to improve the yield of larger-surface-area devices have primarily directed attention to performing the manufacturing steps in the cleanest possible environment, creating masks under extremely expensive manufacturing conditions, and improving mask alignment by use of very sophisticated, precise alignment machines. These areas of attention are extremely expensive, and, as a practical matter, make their use economically unattractive vis-a-vis the final market price which, as a consequence, must be attached to a finished device.
Accordingly, a need remains for a device structure and fabrication process that can produce high voltage solid-state power switches with increased yields in larger size to handle high current but without undesirable parasitic effects.